Electrical detection of selected species

ABSTRACT

The present invention provides an organic field effect transistor and a method of fabricating the transistor. The transistor includes a semiconductive film comprising organic molecules. Probe molecules capable of binding to target molecules are coupled to an outer surface of the semiconductive film such that the interior of the film being substantially free of the probe molecules.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention is directed, in general, to the electricaldetection of selected species, in particular biologically relevantspecies. More specifically, the invention is directed to a biosensordevice that includes an organic field effect transistor and a method ofmanufacturing the device.

BACKGROUND OF THE INVENTION

[0002] There is great interest in the rapid simultaneous detection oflarge numbers of biological species such as naturally occurring DNA,RNA, proteins, and other naturally occurring molecules, as well asman-made aptamers, synthetically modified proteins or toxins. Advancesin biosensor technology have facilitated numerous potential medicalapplications, such as drug discovery, detecting genetic mutations andevaluating the effect of gene therapy or the identification ofbiological toxins.

[0003] For instance, traditional radio-immunoassay approaches todetecting proteins, hormones and various pathogens involve the bindingof antibodies to a solid support to form a micro array and then exposingthe analyte to the array of antibodies. The analysis of DNA fragmentssimilarly involves fixing single stranded target DNA fragments,representing the genome of an organism, for example, to individual wellsin a solid support to form a micro array. Such DNA micro arrays, alsoknown as DNA chips, provide a highly sensitive means of detectingspecific target DNA fragments. The micro array is analyzed by exposingtarget DNA fragments to fluorescently labeled probes of cDNA or mRNA ofunknown identity. When the nucleic acid sequence of the probe cDNA ormRNA is complementary to the nucleic acid sequence of the target DNA,the probe cDNA or mRNA hybridizes to the DNA fragment. The flourescentlabel attached to the cDNA or mRNA is then detected with the aid oflasers and sensitive fluorescence detection equipment.

[0004] The wide-spread application of such DNA micro arrays and othertypes of arrays is limited by a number of factors, however. For example,the micro arrays and the fluorescently labeled cDNA and mRNA probes areexpensive to produce or purchase. The high cost of lasers to initiatefluorescence, detection equipment, such as confocal microscopes andflourescent light detection equipment, also limit wide-spreadapplications of this technology. In addition, the shear bulk of suchequipment limits the physical location where DNA micro arrays can beanalyzed.

[0005] Electrical biosensor devices have been proposed as an alternativemeans for detecting DNA and RNA. An electrical readout corresponding tothe concentration of a target molecule in a particular assay solutionwould allow a substantial reduction in the size and cost of theequipment needed to apply micro array technology. Previous biosensordevices have used an electrode comprising a semiconductive film made oforganic polymers functionalized with selected species of probeoligonucleotides of single stranded nucleic acid sequences.Alternatively, monomers of the organic polymer are functionalized andthen polymerized to form the functionalized organic polymer. In eithercase, the probe oligonucleotides are attached as side chains to theorganic polymers. Semiconductive films are then made of thefunctionalized organic polymers. When exposed to a liquid containing theappropriate complementary target nucleic acid sequence, the probe andtarget nucleic acid sequence hybridize, thereby causing a detectablechange in the conductivity of the functionalized organic polymersincorporated into the semiconductive film.

[0006] One objective of the invention is to provide sensitive electricaldevices for the detection of a variety of target biological species.Another objective of the invention is to provide a method for thefabrication of such devices.

SUMMARY OF THE INVENTION

[0007] The present invention recognizes that the practical applicationof such electric devices has been limited due to poor sensitivity, inpart, because functionalizing the side-chains of the organic polymerreduces the polymer's conductivity. Moreover, the binding of the targetbiomolecules to the probe molecules attached throughout the organicpolymer does not result in a sufficiently large change in conductivityto allow the detection of small quantities of the target biomolecule.

[0008] To address these deficiencies, one embodiment of the presentinvention provides an organic field effect transistor for the detectionof biological target molecules. The transistor includes a semiconductivefilm comprising organic molecules. In addition, a probe molecule iscoupled to an outer surface of the semiconductive film, the film havingan interior substantially free of the probe molecules.

[0009] In another embodiment, the invention further provides a method offabricating an organic field effect transistor for the detection ofbiological target molecules. The method comprises forming a transistorchannel that includes forming a semiconductive film comprising organicmolecules between a source and drain. Forming the transistor channelalso includes coupling a probe molecule to an outer surface of thesemiconductive film, the semiconductive film having an interiorsubstantially free of the probe molecule

[0010] Still another embodiment of the invention is a biosensor systemfor the detection of biological target molecules. The biosensor systemcomprises a biosensor device, a sample, and an assay system. Thebiosensor device includes one or more organic field effect transistors,each of the transistors including a transistor channel. The channel inturn comprises a semiconductive film comprising organic molecules; andprobe molecules coupled to an outer surface of the semiconductive film,such that the semiconductive film has an interior substantially free ofthe probe molecules. The sample is capable of holding a target moleculethat is configured to bind to with one or more of the probe molecules.The assay system is configured to bring the biosensor device in contactwith the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention is best understood from the following detaileddescription, when read with the accompanying FIGUREs. Various featuresmay not be drawn to scale and may be arbitrarily increased or reducedfor clarity of discussion. Reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

[0012]FIG. 1 schematically illustrates a detailed sectional view of anorganic field effect transistor of the present invention;

[0013]FIGS. 2A to 2F schematically illustrate sectional views of anorganic field effect transistor of the present invention at variousstages of manufacture; and

[0014]FIG. 3 schematically illustrates detail sectional views of thebiosensor system of the present invention.

DETAILED DESCRIPTION

[0015] The present invention benefits from the realization that previousbiosensors for detecting biological target molecules have poorconductivity because probe molecules are attached to the interiorside-chains throughout the organic polymers of the semiconductive filmin the biosensor. Probe molecules comprising nuclei acid or amino acidsequences, such as DNA or proteins, impede the formation of orderedsemiconductive films composed of the functionalized organic polymers.This, in turn, detrimentally decreases the efficiency of charge transferbetween the semiconductive organic polymers of the semiconductive film.Moreover, the present invention recognizes that probe molecules, such asDNA, have substantial insulating properties when they are incorporatedthroughout the interior of such films, thereby further decreasing theconductivity of the films.

[0016] The present invention further recognizes the advantages offorming biosensor devices having a semiconductive film that includeselectrically conducting organic molecules with probe molecules attachedsubstantially to the outer surface of the film. The complete orsubstantial absence of probe molecules in the interior of the filmfacilitates the formation of uniform closely packed crystalline orpolycrystalline films of the organic molecules. This is advantageousbecause the charge transfer characteristics of the film depends on theefficient packing of the organic molecules, which generally have one ormore conjugated Pi bonds, to form a system of conjugated Pi bonds. Theefficiency of charge transfer from one organic molecule to anotherincreases as the distance between the conjugated Pi bonding systems ofadjacent organic molecules is decreased.

[0017]FIG. 1 illustrates a schematic representation of a portion of anorganic field effect transistor 100 for the detection of biologicaltarget molecules. The transistor 100 comprises a transistor channel 110having a semiconductive film 115 comprising organic molecules 120. Thetransistor 100 further includes probe molecules 125 coupled to an outersurface 130 of the semiconductive film 115, the film 115 having aninterior substantially free of, if not completely void of, the probemolecules 125. For the purposes of the present invention the termsubstantially free of probe molecules refers to the film's 115 interiorhaving, at most, trace amounts of probe molecules 125 present therein.For instance, in embodiments where the organic molecules 120 arehydrophobic, then hydrophilic probe molecules 125, such as certainnucleic acid or amino acid sequences, will be excluded from the film'sinterior. In such embodiments, the trace amounts of probe molecules 125in the film's interior 115 would be less than or equal to the solubilityof the probe molecule 125 in the organic molecule 120.

[0018] In preferred embodiments, organic molecules 120 have a conjugatedpi system of bonds, although any semiconductive materials having organicmolecules 120 are within the scope of the present invention. Preferably,the organic molecules 120 of the film 115 have an ordered crystalline orpolycrystalline structure. The organic molecules 120 can be any carboncontaining compounds capable of forming a semiconducting film 115. Morepreferably, the organic molecules 120 have a high field effect mobility(i.e., greater than about 10⁻⁴ cm²/V•s). Even more preferably, theorganic molecules 120 have a field effect mobility of greater than about10⁻² cm²/V•s, such as that provided by molecules having a conjugated pisystem, as exemplified by oligothiophene or polythiophene.

[0019] In certain preferred embodiments, the organic molecules 120 areoligomers. Oligomers are more desirable than certain substitutedpolymers due to the better ability of oligomers to form orderedcrystalline films having no probe molecules 125 in their interior. Forthe purposes of the present invention, the term oligomer as applied tothe organic molecules 120 of the semiconductive film 115 referred tomolecules having from 2 to 100 repeating units. The term polymer refersto organic molecules 120 having greater than 100 repeating units. Incertain preferred embodiments the organic molecule 120 is an oligomerhaving from 4 to 20, and even more preferably, from 4 to 10 repeatingunits.

[0020] For example, an oligothiophene has between 2 and 100 repeatingunits of thiophene while polythiophene has greater than 100 repeatingunits of thiophene. In one preferred embodiment, the organic molecule120 is sexithiophene, and more preferably alpha sexithiophene. Othersemiconductive organic compounds, however, are also within the scope ofthe present invention. Nonlimiting examples include oligophenyl orpolyphenyl compounds. The organic molecule 120 may also comprisecombinations of different benzoid aromatic ring structures like benzene,napthalene or anthracene rings coupled to each other such as in such aspentacene, nonbenzoid aromatic rings, or heterocylic rings, such asthiophene.

[0021] The semiconducting film 115 may comprise a single molecular layerof the organic molecule or multiple layers of organic molecules. Thefilm may have a thickness 132 of about 20 Angstroms or higher. In apreferred embodiment, the film 115 has a thickness 132 between about 20and about 100 Angstroms. In certain embodiments where the organicmolecules 120 of the film 115 comprise sexithiophene, the film'sthickness 132 corresponds to one to three molecular layers ofsexithiophene.

[0022] A thin film 115, such as a thickness 132 in the above-citedrange, is preferred because the resulting biosensor device is expectedto be more sensitive. This follows because the current passing throughthe semiconducting film 115 is thought to flow primarily at theinterface 135 between an insulating layer (e.g., a gate dielectric) 140,and the semiconducting film 115. The closer the probe molecules 125 areto that interface 135, the more sensitive the transistor 100 will be tochanges in channel conductivity or channel mobility associated with thebinding of a target molecule 145 to the probe molecule 125. Inparticular, one or both of channel conductivity or channel mobilitychanges due to the binding of target molecules 145 to the probemolecules 125 thereby providing a method for detecting the targetmolecules 145.

[0023] In certain embodiments, where an ultra thin film 115 is desired(e.g., thickness 132 of less than about 30 Angstroms), thesemiconducting film 115 preferably comprises a monolayer of the organicmolecule 120. In some embodiments, it is preferable to covalently attachthe organic molecules 120 to the underlying insulating layer 140. Insuch embodiments, the end of the organic molecule 150 opposite to theend that is coupled to the probe molecule 155 is functionalized tofacilitate covalent attachment to the insulating layer 140. If theinsulating layer 140 is silicon dioxide, for example, the end of theorganic molecule 150 can be functionalized with a silane moiety.

[0024] In still other embodiments, it is advantageous for thesemiconducting film to further include linker molecules 160 coupled tothe end of the organic molecule 155 situated at the outer surface of thefilm 130. One or more functional groups 162, 164 attached to the linkermolecule 160 facilitates the coupling of probe molecules 125 to theorganic molecules 120, as further discussed below. Coupling can involvecovalent or non-covalent interactions between the linker molecule 160and the organic molecule 120 or probe molecule 125. The linker molecule160 may also have one or more spacer groups 166 that serve to separatethe probe molecule from the organic molecule when these molecules arecoupled to each other. A spacer group 166 is desirable in instanceswhere the functional groups 162, 164 used to facilitate coupling ofprobe molecules 125 to the organic molecules 120 could detrimentallyaffect the electrical properties of the semiconducting film 115.Moreover, a spacer group 166, preferably attached to the organicmolecule 120 before deposition, by separating the probe molecule 125from the organic molecule 120, can facilitate a uniform ordered packedstructure so that the film 115 retains its conductive properties afterbeing coupled to the probe molecules 120. In addition, by adjusting thelength of the spacer group 166 it is possible to advantageously increaseor decrease the affect that the binding of the target molecule 145 tothe probe molecule 120 has on the conductive properties of the film 115,for example, by changing the distance between charged target molecules145 and the film 115. Another advantageous feature is that the linkermolecule 160 can provide electrical insulation between thesemiconducting film 115 and the aqueous solution containing the targetmolecules 145. In certain embodiments, therefore, attaching the targetmolecule 145 does not perturb the electrical properties of the organicmolecule 120.

[0025] Examples of suitable linker molecules 160 include compoundshaving amino or thiol functional groups 162 covalently attached to aspacer group 166. In certain preferred embodiments, the spacer group 166is an alkyl chain having up to 20 carbon atoms. In one embodiment, thelinker molecule 160 has an amino functional group 162 attached to oneend of a spacer group 166 of n-hexane. The other end of the n-hexanespacer group 166 is attached to an organic molecule 120 of sexithiopheneat the carbon 5 position in the terminal thiophene ring, as facilitatedby a functional group 164. Alternatively, there may be no functionalgroup 164 on the end of the linker molecule adjacent to the organicmolecule 120. For instance, the n-hexane spacer group 166 can havenon-covalent interactions with the organic molecule 120 that serve tocouple the linker molecule 160 to the organic molecule 120. In otherpreferred embodiments, the linker molecule 160 comprises a layer of anorganic polymer such as polyimide, or an inorganic polymer, such assilicon dioxide. In such embodiments, for example, the linker molecules160 can include an amino or thiol functional group 162 attached to alayer of spacer groups 166 comprising silica formed by depositingamino-silane or thiol-silane on the layer of spacer groups 166.

[0026] The probe molecule 125 can be any molecule capable of beingcoupled to the organic molecules 120 of the semiconductive film 115 andcapable of binding to a specific target molecule 145 or class of targetmolecules 145. In certain preferred embodiments, the probe molecule 125comprises nucleic acid or amino acid sequences, such as DNA or proteins.In other preferred embodiments, the probe molecule 125 is a singlestranded DNA having a nucleic acid sequence that is complementary to atleast a portion of the nucleic sequence of the target molecule 145.Other embodiments of the probe molecule 125 include RNA or aptamers. Instill other embodiments the probe molecule 125 is a protein, such as anantibody or antibody fragment having a high affinity for a targetprotein 125.

[0027] Preferably the target molecule 145 is a biological molecule. Incertain embodiments, the target molecule 145 has a net positive ornegative charge. In certain preferred embodiments the target molecule145 is cDNA or mRNA that is complementary to the probe molecule 125. Insuch embodiments, the target molecule 125 has a net negative charge. Inother embodiments the target molecule 145 is an antigen to the probemolecule 125. When charged target molecules 145 bind to the probemolecules 125 that are coupled to the organic molecules 120 of thesemiconductive film 115, the static charge in the vicinity of thesemiconductive film 115 changes. This, in turn, changes the electricalfield experienced by the semiconductive film 115. As a result of thechange in the electrical field, the flow of current between source anddrain electrodes 170, 175 of the transistor 100 changes in proportion tothe change in the electrical field experienced by the channel 110. Underfixed environmental conditions (e.g., constant pH, temperature and ionicstrength), the extent of change in the electrical field is proportionalto the number of the target molecules 145 that bind to the probemolecules 125.

[0028] In other embodiments however, the target molecule 145 has no netcharge. When a neutral target molecule 145 binds to the probe molecule125, the dielectric constant in the vicinity of the semiconductive film115 changes. This, in turn, changes the capacitance between the fluidcontaining the target molecules 145 and the film 115. This results in achange in the effective gate capacitance which, by changing theelectrical field in the semiconductive film, can be detected as a changein the conductivity of the semiconductor film 115. Consider, forexample, when the target molecule 145 is a protein with a net charge ofzero. When the neutral target protein 145 binds to the probe molecule125, water molecules associated with the probe molecules 125 areexcluded from the semiconductive film's outer surface 130, resulting ina change in dielectric constant in the vicinity of the probe molecule125 and adjacent region of film 145.

[0029] Preferred embodiments of organic field effect transistor 100further include a substrate 180 under the channel 110, a gate 185 overthe substrate 180, with the above mentioned insulating layer 140 overthe gate 185 and the above-mentioned source and drain 170, 175 over theinsulating layer 140, with the channel 110 located between the sourceand drain 170, 175. In certain preferred embodiments, the sensitivity ofthe change in conductivity or mobility of the active channel associatedwith the binding/unbinding of a target molecule 145 to the probemolecule 125 depends on the voltage applied to the gate 185. However,the organic field effect transistor 100 can be configured in other wayswell known to those of ordinary skill in the art.

[0030] In operation, the portion of the transistor above the channel isexposed to an assay solution 190 containing analytes that may includethe target molecule 145. For example, the assay solution 190 may be anaqueous solution that includes buffers, electrolytes and target molecule145. In some embodiments a voltage is applied to the drain 175 and thegate 185 with the source 170 held at ground. In other embodiments avoltage is also applied to the assay solution 190. Preferably theapplied voltage is up to about 10 Volts, and more preferably up to about2 Volts. In one embodiment, the change in electrical field in thevicinity of the channel that is associated with the binding of a targetmolecule 145 to a probe molecule 125 results in a change in the voltagebetween the source and drain, 170, 175 of at least about 0.01 Volts, andmore preferably at least about 0.1 Volts. The corresponding change incurrent between the source and drain, 170, 175 may be about 1000 to 1500times lower in the presence of the target molecule 145 than the currentin the absence of the target molecule 145. One skilled in the art wouldunderstand, however, that the minimal acceptable change in voltagebetween the source and drain, 170, 175 will depend on the smallestcurrent that can be detected. The detection of current changes willinvolve other factors such as the electronic noise floor, devicestability and averaging time.

[0031]FIGS. 2A to 2F illustrate selected steps in another embodiment ofthe present invention, a method of fabricating an organic field effecttransistor 200 for the detection of biological above. Analogous featuresof the transistor 200 are depicted u target molecules, similar to thatdescribed sing similar numerical reference numbers as used in FIG. 1. Asillustrated in FIG. 2A, the method includes providing a conventionalsubstrate 280 comprising, for example, a silicon wafer.

[0032] As illustrated in FIG. 2B, a gate 285 is formed over thesubstrate 280. Suitable conductive gate materials include metals, suchas gold, or conducting polymers, such as doped polythiophene. The gate285 may be deposited by conventional techniques such as chemical vapordeposition, physical sputtering or electron beam evaporation processes.Alternatively, the material used to form the gate 285 may comprise aportion of the substrate, for example silicon doped with a conventionaldopant such as boron or phosphorus thereby making a portion of thesubstrate 280 conductive. As further discussed below, for embodimentswhere multiple organic field effect transistors 200 are desired, thegate material is later patterned using standard photolithographictechniques to form an array of gates, which are not individuallyillustrated.

[0033] As illustrated in FIG. 2C, an insulating layer 240 is thendeposited over the gate 285. Suitable materials for the insulating layer240 include dielectric materials such as silicon dioxide or polyimide.The insulating layer 240 is deposited using conventional processes suchas chemical vapor deposition using tetraethylorthosilicate or thermalgrowth on a doped silicon gate. The insulating layer 240 isconventionally patterned when the individual gates are patterned.

[0034] As illustrated in FIG. 2D, a source and drain 270, 275 are formedover the insulating layer 240. The source and drain 270, 275 cancomprise metals or conducting polymers similar to polymers used for thegate 285. Similar processes as used to form the gate 285 can also beused for depositing the source and drain material and then usingpatterning techniques to form the source the drain regions 270, 275 soas to accommodate one or more channel regions 210 of the device 200.

[0035] As illustrated in FIG. 2E, forming a transistor channel 210includes forming a semiconductive film 215 comprising organic molecules220 between the source and drain 270, 275. Preferably the semiconductingfilm 215 comprising the organic molecule 220 is formed over thesubstrate 280 and more preferably on the insulating layer 240. When theorganic molecule 220 is an oligomer, the film is preferably formed viavacuum sublimation. Typically, vacuum sublimation is conducted atpressures between about 1×10⁻⁴ to 1×10⁻⁶ Torr, using conventionalprocedures well known to those skilled in the art. Vacuum sublimation ispreferred because of the relative simplicity of this procedure. Vacuumsublimation, for example, does not require additional steps tofunctionalize the organic molecule 220 to make it more soluble in asolvent suitable for spin coating.

[0036] If the selected organic molecule 220 is not amenable to vacuumsublimation, and the organic molecule 220 is at least slightly solublein organic solvents, such as chloroform toluene or xylenes, thenalternative conventional procedures such as solution spin coating, vapordeposition or printing may be used. In certain cases it may be desirableto functionalize the organic molecule 220, such as polythiophene, toimprove its solubility in the solvent used for spin coating. An exampleof such functionalization is regio-regular poly(3-hexylthiophene).

[0037] As illustrated in FIG. 2F, forming the channel 210 also includescoupling a probe molecule 225 to an outer surface 230 of thesemiconductive film 215 in a manner that causes the semiconductive film215 to have an interior that is substantially free of the probemolecules 225. Coupling between the probe molecule 225 and the organicmolecule 220 of the film 215 is achieved using any number ofconventional methods well known to those of ordinary skill in the art.Exemplary methods include those described by Korri-Youssoufi, H., etal., J. Am. Chem. Soc. 119:7388-89 (1997) and Katz, H. E., et al. Chem.Mater. 10:633-38 (1998), which are incorporated herein in theirentirety.

[0038] For instance, a functional group 264 can be attached to one end255 of the organic molecule 220 to facilitate coupling to the probemolecule 225. In certain instances, it may be desirable to protect thefunctional group 264 attached to the organic molecule 220 during theformation of the film 215. Afterwards, the functional group 264 isde-protected to expose the functional group for reaction with the probemolecule 225. Analogous considerations apply to the attachment of afunctional group 262 to the probe molecule 225, and the inclusion of anoptional spacer group 266 between the functional groups.

[0039] Consider an embodiment where, for example, the organic molecule220 is sexithiophene and the probe molecule 225 is a single strandedoligmer of DNA or RNA. The probe DNA or RNA 225 can be coupled to thefilm 215 by reacting an amino functional group 262 attached to one end255 of the organic molecules 220 of the film 215 to acidic groups of theprobe DNA or RNA 225 to form an amide bond. Alternatively, a thiolfunctional group 264 attached to one end 255 of the organic molecule 220can react with a thiol group present in the probe molecule 225 to form adisulphide bond that couples the probe molecule 225 to the film 215. Asindicated above, the thiol group 264 may be protected via acetylation toproduce a thiol ester (e.g., CH₃—CO—S—R, where R is the organic molecule240 or a linker group 260 coupled to the organic molecule 240 asdiscussed above) during the formation of the film 215, and thende-protected by exposure to ammonium hydroxide.

[0040] In some embodiments, it is preferable for the coupling betweenthe probe molecules 225 and semiconductive film 215 to be carried out ina solvent that the organic molecules 220 of the film 215 are not solublein. Examples of suitable solvents include water, or organic solventslike ethanol and acetonitrile. The use of such organic solvents helps toprevent diffusion of the probe molecules 225 into the interior of thefilm 215 during the reaction to couple the probe molecules 225 to theouter surface of the film 230. In certain embodiments it is desirablefor the organic molecule 220 to be an oligomer such as sexithiophene,because oligomers tend to be less soluble in such organic solvents ascompared to polymers such as polythiophene.

[0041] As noted above, forming the channel 210 includes coupling theprobe molecule 225 to the outer surface of the semiconductive film 230.By forming the film 215 of organic molecules 220 prior to coupling theprobe molecules 225 to the organic molecules 220, one can ensure thatthe interior of the film 215 is substantially free of probe molecules225. Preferably, the probe molecules 225 couple to the organic molecules220 situated at the outer surface of the film 230. More preferably, theprobe molecules 225 are coupled to the ends of the organic moleculesthat are closest to the external surface of the film 230. In certainpreferred embodiments, as further discussed below, there are multipletransistors 200 each having different probe molecules 225 coupled totheir associated channels 210, thereby forming an array of differentfield effect transistors 200 fabricated on a single substrate 280. Insuch embodiments, it is desirable to use ink-jet printing technology ormicrofluidic devices to deposit the different probe molecules 225 on thechannels 210 located in the array.

[0042]FIG. 3 illustrates yet another embodiment of the presentinvention, a biosensor system 300 for the detection of biological targetmolecules. The system 300 includes a biosensor device 310 that includesa plurality of organic field effect transistors 320. The transistors 320have a channel 325 comprising a semiconductive film and probe moleculescoupled to an outer surface of the semiconductive film, as presentedabove. In certain embodiments, the transistors 320 have different typesof probe molecules in each channel 325 that are capable of binding todifferent types of target molecules 330. Any of the above-discussedembodiments of the transistor 320 and target molecules 335 discussedabove may be included in the system 300.

[0043] The system further includes an assay system 340 configured tobring the biosensor device 310 in contact with the sample 330. The assaysystem 340 may comprise a manual or robotically operated fluidicworkstation having multiple print heads, well known to those skilled inthe art, for loading probe molecules and samples into the transistors ofthe biosensor. More preferably, as illustrated in FIG. 3, the assaysystem 340 comprises a micro fluidic network 345 coupled to thetransistors 320, the network 345 capable of electrically isolating thechannels of the transistors from remaining components of the transistor,such as the source and drain 350, 355 of the transistor. In otherpreferred embodiments, the micro fluidic network 345 has conduits 360that direct multiple samples to discrete transistors 320 of thebiosensor device 310 for simultaneous or parallel analysis of thesamples. Such networks may comprise elastomeric silicones, such aspolydimethylsiloxane, and are formed using procedures well-known tothose skilled in the art, e.g., as described by Thorsen et al., Science298:580-584 (2002), incorporated herein in its entirety.

[0044] Although the present invention has been described in detail,those of ordinary skill in the art should understand that they can makevarious changes, substitutions and alterations herein without departingfrom the scope of the invention.

What is claimed is:
 1. An organic field effect transistor for the detection of biological target molecules comprising: a transistor channel having a semiconductive film comprising organic molecules and probe molecules coupled to an outer surface of said semiconductive film, said film having an interior substantially free of said probe molecules.
 2. The transistor as recited in claim 1, wherein said organic molecules have a conjugated pi bond.
 3. The transistor as recited in claim 1, wherein said organic molecules are selected from the group consisting of polythiophene and oligothiophene.
 4. The transistor as recited in claim 1, wherein said semiconductive film is crystalline or polycrystalline.
 5. The transistor as recited in claim 1, wherein said semiconductive film comprises a monolayer of said organic molecules, wherein one end of said organic molecules is coupled to an gate dielectric located below said semiconductive film.
 6. The transistor as recited in claim 1, wherein said semiconductive film further includes a linker molecule coupled to said organic molecule at said outer surface and coupled to one of said probe molecules.
 7. The transistor as recited in claim 1, wherein said transistor channel is an active channel, and wherein said probe molecules are capable of binding to target molecules and are chemical bonded to said outer surface.
 8. The transistor as recited in claim 1, wherein said probe molecules are selected from the group consisting of: single stranded DNA; RNA; aptamers; and proteins, and are configured to bind to biological target molecules selected from the group consisting of: cDNA; mRNA; and antibodies.
 9. The transistor as recited in claim 1, wherein binding a target molecule to said probe molecule results in a change in a conductivity or mobility of said channel.
 10. A method of fabricating an organic field effect transistor for the detection of biological target molecules comprising: forming a transistor channel including: forming a semiconductive film comprising organic molecules between a source and drain; and coupling a probe molecule to an outer surface of said semiconductive film, said semiconductive film having an interior substantially free of said probe molecule.
 11. The method as recited in claim 10, wherein said semiconductive film is polycrystalline or crystalline.
 12. The method as recited in claim 10, wherein said semiconductive film is formed by depositing said organic molecules by a process selected from the group consisting of: spin coating; casting; vapor deposition and printing.
 13. The method as recited in claim 10, wherein said organic molecules are functionalized to improve a solubility of said organic molecule in an organic solvent prior to said forming.
 14. The method as recited in claim 10, wherein said organic molecules further include a functional group capable of facilitating said coupling.
 15. The method as recited in claim 10, wherein said coupling is carried out in an organic solvent selected from the groups consisting of chloroform; toluene; and xylene.
 16. The method as recited in claim 14, wherein said probe molecules are coupled to ends of said organic molecules that are closest to said exterior surface of said film.
 17. The method as recited in claim 10, wherein binding a target molecule to said probe molecule results in a change in a conductivity or mobility of said channel.
 18. A biosensor system for the detection of biological target molecules comprising: a biosensor device, including: one or more organic field effect transistors each of said transistors including a channel comprising: a semiconductive film comprising organic molecules; and probe molecules capable of binding to target molecules coupled to an outer surface of said semiconductive film, said semiconductive film having an interior substantially free of said probe molecules; a sample capable of holding said target molecules; and an assay system configured to bring said biosensor device in contact with said sample.
 19. The biosensor system as recited in claim 18, wherein said assay system comprises a micro fluidic network coupled to said transistors, said network capable of electrically isolating said channel from remaining components of said transistors.
 20. The biosensor system as recited in claim 19, wherein said micro fluidic network is configured to direct multiple said sample to individual ones of said transistors of said device for parallel analysis of said samples. 